1. Field of the Invention
The present invention relates to a bonded substrate stack, a method of manufacturing the bonded substrate stack, and a method of manufacturing a substrate such as an SOI substrate using the bonded substrate stack.
2. Description of the Related Art
A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.
(1) The integration degree can be increased because dielectric isolation is easy.
(2) The radiation resistance can be increased.
(3) The operating speed of the device can be increased because the stray capacitance is small.
(4) No well step is necessary.
(5) Latch-up can be prevented.
(6) A complete depletion type field effect transistor can be formed by thin film formation.
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As one SOI technology, the SOS (Silicon On Sapphire) technology by which Si is heteroepitaxially grown on a single-crystal sapphire substrate by CVD (Chemical Vapor Deposition) has been known for a long time. This SOS technology once earned a reputation as the most matured SOI technology. However, the SOS technology has not been put into practical use to date because, e.g., a large amount of crystal defects are produced by lattice mismatch in the interface between the Si layer and the underlying sapphire substrate, aluminum that forms the sapphire substrate mixes in the Si layer, the substrate is expensive, and it is difficult to obtain a large area.
Attempts have recently been made to realize the SOI structure without using any sapphire substrate. The attempts are roughly classified into two methods.
In the first method, the surface of a single-crystal Si substrate is oxidized, and a window is formed in the oxide film (SiO2 layer) to partially expose the Si substrate. Single-crystal Si is epitaxially grown laterally using the exposed portion as a seed, thereby forming a single-crystal Si layer on SiO2 (in this method, an Si layer is deposited on an SiO2 layer).
In the second method, a single-crystal Si substrate itself is used as an active layer, and an SiO2 layer is formed on the lower surface of the substrate (in this method, no Si layer is deposited).
As a means for realizing the first method, a method of directly epitaxially growing single-crystal Si in the horizontal direction from the single-crystal Si layer by CVD (CVD), a method of depositing amorphous Si and epitaxially growing single-crystal Si laterally in the solid phase by annealing (solid phase epitaxial growth), a method of irradiating an amorphous silicon layer or a polysilicon layer with a focused energy beam such as an electron beam or laser beam to grow a single-crystal Si layer on an SiO2 layer by melting recrystallization (beam annealing), or a method of scanning band-shaped melting regions by a rod-like heater (zone melting recrystallization) is known.
All of these methods have both advantages and disadvantages and many problems of controllability, productivity, uniformity, and quality, and therefore have not been put into practical use in terms of industrial applications. For example, CVD requires sacrifice oxidation to form a flat thin film. Solid phase epitaxial growth is poor in crystallinity. In beam annealing, the process time required to scan the focused beam and controllability for beam superposition or focal point adjustment pose problems. Zone melting recrystallization is the most matured technique, and relatively large-scaled integrated circuits have been fabricated on a trial basis. However, since a number of crystal defects such as a subboundary undesirably remain, minority carrier devices cannot be created.
As the above second method, i.e., as the method without using the Si substrate as a seed for epitaxial growth, the following four techniques can be used.
As the first technique, an oxide film is formed on a single-crystal Si substrate having a V-shaped groove formed in the surface by anisotropic etching. A polysilicon layer having nearly the same thickness as that of the single-crystal Si substrate is deposited on the oxide film. After this, the single-crystal Si substrate is polished from the lower surface, thereby forming, on the thick polysilicon layer, a substrate having a single-crystal Si region surrounded and dielectrically isolated by the V-shaped groove. With this technique, a substrate having satisfactory crystallinity can be formed. However, there are problems of controllability and productivity in association with the process of depositing polysilicon as thick as several hundred micron or the process of polishing the single-crystal Si substrate from the lower surface to leave the isolated Si active layer.
The second technique is SIMOX (Separation by Ion Implanted Oxygen). In this technique, oxygen ions are implanted into a single-crystal Si substrate to form an SiO2 layer. In this technique, to form an SiO2 layer in a substrate, oxygen ions must be implanted at a dose of 1018 (ions/cm2) or more. This implantation takes a long time to result in low productivity and high manufacturing cost. In addition, since a number of crystal defects are generated, the quality is too low to manufacture minority carrier devices.
As the third technique, an SOI structure is formed by dielectric isolation by oxidizing a porous Si layer. In this technique, an n-type Si island is formed on the surface of a p-type single-crystal Si substrate by proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or epitaxial growth and patterning. This substrate is anodized in an HF solution to convert only the p-type Si substrate around the n-type Si island into a porous structure. After this, the n-type Si island is dielectrically isolated by accelerated oxidation. In this technique, since the Si region to be isolated must be determined before the device process, the degree of freedom in device design is limited.
As the fourth technique, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this technique, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
Selective etching is effective to uniformly thin the substrate. However, the selectivity ratio is as low as about 102, the surface planarity after etching is poor, and the crystallinity of the SOI layer is unsatisfactory.
A transparent substrate represented by a glass substrate is important in forming a contact sensor as a light-receiving element or a projection liquid crystal display device. To realize highly precise pixels (picture elements) having higher density and resolution for the sensor or display device, a high-performance driving element is required. For this purpose, a demand has arisen for a technique of forming a single-crystal Si layer having excellent crystallinity on a transparent substrate.
However, when an Si layer is deposited on a transparent substrate represented by a glass substrate, only an amorphous Si layer or a polysilicon layer is obtained. This is because the transparent substrate has an amorphous crystal structure, and the Si layer formed on the substrate reflects the disorderliness of the crystal structure of the transparent substrate.
The present applicant has disclosed a new SOI technology in Japanese Patent Laid-Open No. 5-21338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred xc3x85 to 10-xcexcm thick SOI films can be manufactured by a single manufacturing apparatus.
The present applicant has also disclosed, in Japanese Patent Laid-Open No. 7-302889, a technique of bonding first and second substrates, separating the first substrate from the second substrate without breaking the first substrate, smoothing the surface of the separated first substrate, forming a porous layer again on the first substrate, and reusing this substrate. Since the first substrate is not wasted, this technique is advantageous in largely reducing the manufacturing cost and simplifying the manufacturing process.
To separate the bonded substrate stack into two substrates without breaking the first and second substrates, for example, the two substrates are pulled in opposite directions while applying a force in a direction perpendicular to the bonding interface, a shearing force is applied parallel to the bonding interface (for example, the two substrates are moved in opposite directions in a plane parallel to the bonding interface, or the two substrates are rotated in opposite directions while applying a force in the circumferential direction), a pressure is applied in a direction perpendicular to the bonding interface, a wave energy such as an ultrasonic wave is applied to the separation region, a peeling member (e.g., a sharp blade such as knife) is inserted into the separation region parallel to the bonding interface from the side surface of the bonded substrate stack, the expansion energy of a substance filling the pores of the porous layer functioning as the separation region is used, the porous layer functioning as the separation region is thermally oxidized from the side surface of the bonded substrate stack to expand the volume of the porous layer and separate the substrates, or the porous layer functioning as the separation region is selectively etched from the side surface of the bonded substrate stack to separate the substrate.
Porous Si was found in 1956 by Uhlir et al. who were studying electropolishing of semiconductors (A. Uhlir, Bell Syst. Tech. J., vol. 35, 333 (1956)). Porous Si can be formed by anodizing an Si substrate in an HF solution.
Unagami et al. studied the dissolution reaction of Si upon anodizing and reported that holes were necessary for anodizing reaction of Si in an HF solution, and the reaction was as follows (T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)).
Si+2HF+(2xe2x88x92n)e+xe2x86x92SiF2+2H+nexe2x88x92
SiF2+2HFxe2x86x92SiF4+H2
SiF4+2HFxe2x86x92H2SiF6
or
Si+4HF+(4xe2x88x92xcex)e+xe2x86x92SiF4+4H+xcexexe2x88x92
SiF4+2HFxe2x86x92H2SiF6
where e+ and exe2x88x92 each represent a hole and an electron, respectively, and n and xcex are the number of holes necessary to dissolve one Si atom. According to them, when n greater than 2 or xcex greater than 4, porous Si is formed.
The above fact suggests that p-type Si having holes is converted into porous Si while n-type Si is not converted. The selectivity in this conversion has been reported by Nagano et at. and Imai (Nagano, Nakajima, Anno, Onaka, and Kajiwara, IEICE Technical Report, vol. 79, SSD79-9549 (1979)), (K. Imai, Solid-State Electronics, vol. 24, 159 (1981)).
However, it has also been reported that n-type at a high concentration is converted into porous Si (R. P. Holmstrom and J. Y. Chi, Appl. Phys. Lett., vol. 42, 386 (1983)). Hence, it is important to select a substrate which can be converted into a porous Si substrate independently of p- or n-type.
To form a porous layer, in addition to anodization, ions may be implanted into a silicon substrate.
For example, in the method described in Japanese Patent Laid-Open No. 5-21338, i.e., the method in which a substrate (to be referred to as a bonded substrate stack) obtained by bonding a first substrate having a non-porous layer such as a single-crystal Si layer on a porous layer to a second substrate via an insulating layer is separated at the porous layer, thereby transferring the non-porous layer formed on the first substrate side to the second substrate, the technique of separating the bonded substrate stack is very important.
For example, in separating the bonded substrate stack, if it is separated at a portion except the porous layer as the separation layer, the non-porous layer (e.g., a single-crystal Si layer) to be used as an active layer is broken, and no desired SOI substrate can be obtained.
The present invention has been made in consideration of the above situation, and has as its object to provide a bonded substrate stack capable of appropriately being separated at a porous layer, a method of manufacturing the bonded substrate stack, and a method of manufacturing a substrate such as an SOI substrate using the bonded substrate stack.
According to the first aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the first step of preparing a first substrate having a porous layer inside, a first layer on the porous layer, and a second layer on the first layer, the second step of bonding a major surface of the first substrate to a second substrate to prepare a bonded substrate stack, and the third step of chemically processing the bonded substrate stack to make at least part of an outer peripheral edge of the first layer retreat toward the inside of the bonded substrate stack.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the third step preferably comprises, e.g., chemically processing the bonded substrate stack to obtain a structure in which at least part of the outer peripheral edge of the first layer of the bonded substrate stack is located at or inside an outer peripheral edge of a region where the first substrate and the second substrate are bonded.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the third step preferably comprises, e.g., the step of oxidizing at least part of the outer peripheral portion of the first layer of the bonded substrate stack prepared in the second step.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the third step preferably comprises, e.g., the step of etching at least part of the outer peripheral edge of the first layer of the bonded substrate stack prepared in the second step.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first layer is, e.g., a semiconductor layer.
In the bonded substrate stack manufacturing method according to first aspect of the present invention, the first layer is, e.g., an Si layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first layer is, e.g., a single-crystal Si layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first layer is, e.g., a compound semiconductor layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first step preferably comprises, e.g., the step of anodizing an Si substrate to form the porous layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first step preferably comprises, e.g., the step of implanting ions into an Si substrate to form the porous layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, for example, the first layer is an Si layer, and the second layer is an SiO2 layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the first step preferably comprises, e.g., the step of forming an Si layer serving as the first layer on the porous layer, and the step of thermally oxidizing a surface of the Si layer to form an SiO2 layer serving as the second layer on the Si layer.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the second substrate is, e.g., one of an Si substrate and a substrate having an SiO2 layer on a surface of an Si substrate.
In the bonded substrate stack manufacturing method according to the first aspect of the present invention, the second substrate is, e.g., one of a transparent substrate and an insulating substrate.
According to the second aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the first step of preparing a first substrate having a porous layer inside, a first layer in a predetermined region on the porous layer, and a second layer that covers an upper surface and at least part of a side surface of the first layer, and the second step of bonding a major surface of the first substrate to a second substrate to prepare a bonded substrate stack.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first step preferably comprises, e.g., after the second step, forming the first layer and the second layer to obtain a structure in which at least part of an outer peripheral edge of the first layer is located inside an outer peripheral edge of a region where the first substrate and the second substrate are bonded.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first step preferably comprises, e.g., the step of forming a layer of a predetermined material on a substantially entire surface of the porous layer and pattering the layer to form the first layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first step preferably comprises, e.g., the step of growing the first layer having a predetermined shape on the porous layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first layer is, e.g., a semiconductor layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first layer is, e.g., an Si layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first layer is, e.g., a single-crystal Si layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first layer is, e.g. a compound semiconductor layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first step preferably comprises, e.g., the step of anodizing an Si substrate to form the porous layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, for example, the first layer is a n Si layer, and the second layer is a n SiO2 layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the first step preferably comprises, e.g., the step of forming an Si layer serving as the first layer on the porous layer, and the step of thermally oxidizing a surface of a substrate having the Si layer to form an SiO2layer serving as the second layer.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the second substrate is, e.g., one of an Si substrate and a substrate having an SiO2 layer on a surface of an Si substrate.
In the bonded substrate stack manufacturing method according to the second aspect of the present invention, the second substrate is, e.g., one of a transparent substrate and an insulating substrate.
According to the third aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the first step of preparing a first substrate having a porous layer inside, a first layer on a portion other than an outer peripheral portion of the porous layer, and a second layer that covers an upper surface of the first layer, and the second step of bonding a major surface of the first substrate to a second substrate to prepare a bonded substrate stack.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first step preferably comprises, e.g., preparing the first substrate having a structure in which the position of an outer peripheral edge of the first layer substantially matches that of an outer peripheral edge of the second layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first step preferably comprises, e.g., the step of forming a lower layer of a first material on a substantially entire surface of the porous layer, the step of forming an upper layer of a second material on a substantially entire surface of the lower layer, the step of removing an outer peripheral portion of the upper layer to form the second layer, and the step of removing an outer peripheral portion of the lower layer using the second layer as a mask pattern to form the first layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first step comprises the step of forming an Si layer on a substantially entire surface of the porous layer, the step of thermally oxidizing a surface of a substrate having the Si layer to form an SiO2 layer, the step of removing an outer peripheral portion of the SiO2layer to form the second layer, and the step of removing an outer peripheral portion of the Si layer using the second layer as a mask pattern to form the first layer.
The bonded substrate stack manufacturing method according to the third aspect of the present invention preferably further comprises, e.g., the third step of chemically processing the bonded substrate stack prepared in the second step to locate at least part of an outer peripheral edge of the first layer at or inside an outer peripheral edge of a region where the first substrate and the second substrate are bonded.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the third step preferably comprises, e.g., the step of oxidizing an outer peripheral portion of the bonded substrate stack prepared in the second step.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first layer is, e.g., a semiconductor layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first layer is, e.g., an Si layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first layer is, e.g., a single-crystal Si layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first layer is, e.g., a compound semiconductor layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the first step preferably comprises, e.g., the step of anodizing an Si substrate to form the porous layer.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the second substrate is, e.g., one of an Si substrate and a substrate having an SiO2 layer on a surface of an Si substrate.
In the bonded substrate stack manufacturing method according to the third aspect of the present invention, the second substrate is, e.g., one of a transparent substrate and an insulating substrate.
According to the fourth aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the first step of preparing a first substrate having a porous layer inside, and a first layer on the porous layer, the second step of bonding a major surface of the first substrate to a second substrate to prepare a bonded substrate stack, and the third step of chemically processing the bonded substrate stack to make at least part of an outer peripheral edge of the first layer retreat toward the inside of the bonded substrate stack.
In the bonded substrate stack manufacturing method according to the fourth aspect of the present invention, the third step preferably comprises, e.g., the step of oxidizing at least part of the outer peripheral portion of the first layer of the bonded substrate stack prepared in the second step.
In the bonded substrate stack manufacturing method according to the fourth aspect of the present invention, the third step preferably comprises, e.g., the step of etching at least part of the outer peripheral edge of the first layer of the bonded substrate stack prepared in the second step.
According to the fifth aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the first step of preparing a first substrate having a porous layer inside, and a first layer on a portion other than an outer peripheral portion of the porous layer, and the step of bonding a major surface of the first substrate to a second substrate.
According to the sixth aspect of the present invention, there is provided a method of manufacturing a substrate, comprising the step of preparing a bonded substrate stack by one of the above bonded substrate stack manufacturing methods, and the step of separating the prepared bonded substrate stack into two substrates at the porous layer.
The bonded substrate stack manufacturing method according to the sixth aspect of the present invention preferably further comprises, e.g., the step of removing a porous layer remaining on a surface on a second substrate side of the two substrates separated in the separation step.
The bonded substrate stack manufacturing method according to the sixth aspect of the present invention preferably further comprises, e.g., the step of removing a porous layer remaining on a first substrate side of the two substrates separated in the separation step to enable reuse of the first substrate.
In the bonded substrate stack manufacturing method according to the sixth aspect of the present invention, the separation step preferably comprises, e.g., ejecting a fluid to a portion near a bonding interface of the bonded substrate stack and separating the bonded substrate stack into two substrates at the porous layer by the fluid.
According to the seventh aspect of the present invention, there is provided a bonded substrate stack having a structure in which a major surface of a first substrate having a porous layer inside, a first layer on the porous layer, and a second layer on the first layer is bonded to a second substrate, wherein at least part of an outer peripheral portion of the bonded substrate stack, an outer peripheral edge of the first layer is separated from an outer peripheral edge of the bonded substrate stack by a predetermined distance toward the inside.
According to the eighth aspect of the present invention, there is provided a bonded substrate stack having a structure in which a major surface of a first substrate having a porous layer inside, a first layer On the porous layer, and a second layer on the first layer is bonded to a second substrate, wherein at least part of an outer peripheral portion of the bonded substrate stack, an outer peripheral edge of the first layer is located at or inside an outer peripheral edge of the second layer.
According to the ninth aspect of the present invention, there is provided a bonded substrate stack having a structure in which a major surface of a first substrate having a porous layer inside, a first layer on the porous layer, and a second layer on the first layer is bonded to a second substrate, wherein at least part of an outer peripheral portion of the bonded substrate stack, an outer peripheral edge of the first layer is located at or inside an outer peripheral edge of a region where the first substrate and the second substrate are bonded.
According to the 10th aspect of the present invention, there is provided a bonded substrate stack having a structure in which a major surface of a first substrate having a porous layer inside and a first layer on the porous layer is bonded to a second substrate, wherein at least part of an outer peripheral portion of the bonded substrate stack, an outer peripheral edge of the first layer is separated from an outer peripheral edge of the bonded substrate stack by a predetermined distance toward the inside.
According to the 11th aspect of the present invention, there is provided a method of manufacturing a bonded substrate stack, comprising the process step of chemically processing a bonded substrate stack in which a major surface of a first substrate having a porous layer inside, a first layer on the porous layer, and a second layer on the first layer is bonded to a second substrate to make at least part of an outer peripheral edge of the first layer retreat toward the inside of the bonded substrate stack.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the process step preferably comprises, e.g., chemically processing the bonded substrate stack to obtain a structure in which at least part of the outer peripheral edge of the first layer of the bonded substrate stack is located at or inside an outer peripheral edge of a region where the first substrate and the second substrate are bonded.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the process step preferably comprises, e.g., the step of oxidizing at least part of the outer peripheral portion of the first layer of the bonded substrate stack.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the process step preferably comprises, e.g., the step of etching at least part of the outer peripheral edge of the first layer of the bonded substrate stack.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the first layer is preferably, e.g., a semiconductor layer.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the first layer is preferably, e.g., an Si layer.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the first layer is preferably, e.g., a single-crystal Si layer.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the first layer is preferably, e.g., a compound semiconductor layer.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the porous layer of the first substrate is preferably, e.g., a porous layer formed by anodizing an Si substrate.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the porous layer of the first substrate is preferably, e.g., a porous layer formed by implanting ions into an Si substrate.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, preferably, for example, the first layer is an Si layer, and the second layer is an SiO2 layer.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the second substrate is preferably, e.g., one of an Si substrate and a substrate having an SiO2 layer on a surface of an Si substrate.
In the bonded substrate stack manufacturing method according to the 11th aspect of the present invention, the second substrate is preferably, e.g., one of a transparent substrate and an insulating substrate.